The present invention relates to a process for the preparation of alkylphosphonous acid esters and to the use of the products prepared by this process.
Alkylphosphonous acid esters are valuable synthetic building blocks which can be employed, for example, for the preparation of crop protection agents and flame retardants.
The preparation of these compounds is technically complex and has hitherto been carried out, in the case of the particularly interesting esters of methylphosphonous acid, from the corresponding phosphonous acid dihalides by reaction with alcohols or by hydrolysis of the phosphonous acid dihalides to the phosphonous acids followed by esterification.
The phosphonous acid dihalides on which both processes are based, such as, for example, methyldichlorophosphine, which is extremely difficult to handle, have hitherto been prepared in a complex synthesis from phosphorus trihalides and alkyl halides in the presence of aluminum chloride (Houben-Weyl, Volume 12/1, p. 306). The reaction is highly exothermic and can only be controlled with difficulty in industry. In addition, various by-products are formed which, like some of the above-mentioned starting materials, are toxic, self-igniting and/or corrosive, i.e. are highly undesirable.
There is therefore a need for a process for the preparation of alkylphosphonous acid esters which can be carried out in a simple manner and in which uniform products are obtained in high yield. A process of this type should also be significantly superior to the processes known hitherto in economic and environmental terms.
The invention thus has the object of providing a process for the preparation of alkylphosphonous acid esters which avoids the above-mentioned disadvantages and, in particular, starts from easily handled, industrially available and readily controllable starting materials.